The desk-top analyzers introduced by Eastman Kodak Company in 1986 under the trademark "Ektachem DTSC" analyzer calculated rate reactions using a screening scheme shown in FIG. 1 of this application, except that during the "early window", not only were the initial rates ascertained, but also the predicted concentrations as a required step. E.g., DD.sub.s /DT.sub.s predicted a concentration of C.sub.s. Such predicted concentrations were then checked against a known high concentration C.sub.m (corresponding to the curves shown). If the value of C.sub.s was within the range of C.sub.1 through C.sub.m, then the analyzer used all the density readings in the early window, along with a portion of the density readings in the later window, to determine the rate for the following reasons:
Low concentration samples tend to have linear curves during the early window as well as a portion of the later window, producing rates which distinguish between sample concentrations C.sub.1 through C.sub.m. However, in the event the predicted concentration C.sub.s was beyond C.sub.m, in other words, too large a concentration, then the analyzer made the "final" rate readings only during the early window. The reason is that concentrations larger than C.sub.m had rates that flattened out so much during the later window that variations on those larger concentrations were indistinguishable from each other and produced rates similar to those found for low concentration samples.
Although that procedure works well with rate reactions conducted on the "DTSC" analyzer, it creates a problem when assaying certain immuno-rate chemistries such as digoxin and phenytoin. The latter have rapid changes in the rate curve which, we have discovered, produces a consistent bias in the results achieved by that method. Therefore, there has been a need prior to this invention to develop an initial screening program that does not suffer from this problem, when applied to these immunorate chemistries.